Field of the Invention:
The invention relates to a method for electrically connecting a semiconductor chip to at least one contact surface through the use of a thin wire having a first end welded to the at least one contact surface and a second end welded to a metal piece disposed on a contact zone of the semiconductor chip and connected thereto in a conductive manner.
Such methods are carried out in a known way by bonding. It is customary in such a case to guide the gold wire, which is usually about 24 .mu.m thick, in a capillary, at the end of which a piece of the gold wire juts out. The piece jutting out is melted through the use of a hydrogen flame or an electrically ignited spark, with the result that the piece is formed into a small ball. The ball is then pressed onto a first contact surface through the use of the capillary and in the process is connected to the contact surface as a result of capillary vibrations in the ultrasonic range, producing a form of the gold wire which is designated as a nailhead. In the case of so-called thermo-sonic bonding, the contact surface is additionally heated to a temperature of about 200.degree. C. to 300.degree. C. Subsequently, the capillary is moved upwards again and is guided in an arc, the so-called loop, to a second contact surface which is to be connected to the first contact surface. At that point the capillary is lowered again and pinches off the gold wire on the second contact surface. The relatively high pressure produces a good connection between the gold wire and the second contact surface, with the resulting wedge structure being designated as a wedge. That type of bonding is also referred to as ball-wedge bonding, in contrast to wedge-wedge bonding of the type which is customary with aluminum wires.
When connecting semiconductor chips to contact surfaces, the ball is usually connected to the semiconductor chip and the wedge to the contact surface, since the ball can be better positioned and the resulting nailhead has a more precisely defined size than the wedge. In addition, the risk of destroying structures on the semiconductor chip is very low in the ball-on-chip method. However, that procedure has the disadvantage that the loop turns out to be relatively large, although that is not important with customary semiconductor housings.
However, in the case of smart cards, for example, the housing height, which corresponds to the thickness of the smart card, is very small and fixedly predetermined. If in that case a component, for example the carrier, is of a somewhat thicker construction, that additional space requirement must be saved again by a reduction at another point.
It is known from German Published, Non-Prosecuted Patent Application DE 36 21 917 Al to connect the wire first of all to a contact surface and then to the semiconductor chip. Since that procedure provides the nailhead contact on the contact surface and the wedge contact on the contact zone of the semiconductor chip, a small loop height is produced. However, there is the risk of a short circuit with the chip surface due to the shallow angle at which the wire is guided onto the contact zone of the chip. In addition, the proportion of gold in the gold/aluminum joining zone (contact zones of semiconductor chips usually are formed of aluminum) is distinctly less than in a nailhead contact. As a result, the gold is demonstrably converted completely into brittle intermetallic phases, resulting in contact detachment or contact isolation.
European Patent Application 0 397 426 A2 discloses a configuration in which, before a wedge contact is provided on a contact surface of the semiconductor chip, a nailhead contact is applied to that contact surface and the wedge contact is first provided on that nailhead contact. That increases the reliability of the connection.
The nailhead contact yields the same metallurgical conditions as is the case with traditional thermo-sonic nailhead contact-making.
However, the planar guidance of the bonding wire over the chip surface, particularly when the contact zone is situated in the inner region of the chip surface, may result in contact between the bonding wire and the edge of the semiconductor chip, which can lead to a short circuit.
Japanese Patent Abstract 6-97350 (A) from Patent Abstracts of Japan, E-1574 Jul. 6, 1994, Vol. 18, No. 359 discloses an electrical connection of a semiconductor chip to a lead frame which has a contact element in the form of a wedge, to which a wedge contact is applied in order to achieve better guidance of the bonding wire. However, the contact element in the form of a wedge is an individual part which is complicated to produce and must be disposed on the lead frame in a special method step.